To acquire analog signals delivered by microsensors, use is made of acquisition devices employing switched capacitors or a charge amplifier. The acquisition devices with switched capacitors constitute a first family of devices which make it possible to take a direct measurement of the electrical charges produced at the terminals of a microsensor by using charge transfers between terminals of capacitors that are incorporated in these devices. The devices with switched capacitors can be produced on ASICs (Application-Specific Integrated Circuits). The devices belonging to this first family are dedicated to the acquisition of signals originating from a single type of microsensor and are, moreover, relatively costly to develop. The acquisition devices with charge amplifier constitute a second family of devices which provide a simple technical solution for indirectly acquiring signals delivered by microsensors. For these devices, acquisition entails measuring an amplitude modulation produced on a carrier by microsensors, for example capacitive-type microsensors, which create, in response to a stimulus, an apparent variation of capacitance producing an amplitude modulation on a carrier. Microgyrometers and microaccelerometers etched on a silicon wafer, on board aircraft, are capacitive-type microsensors.
When idle, that is, when no stimulus is applied to them, the capacitive microsensors present, at their terminals, a capacitance of non-zero value, called “spurious capacitance”, denoted CSpurious. It can be considered, as is represented in an equivalent electronic diagram of a microsensor 1 in FIG. 1, that the spurious capacitance is in parallel with a second capacitance, called “useful” capacitance and denoted CUseful. The changes to the useful capacitance constitute an image of the stimulus that you want to measure. To avoid biasing a measurement of the useful capacitance it is essential to compensate for the effects of the spurious capacitance on the signal delivered by the microsensor.
One solution of the state of the art, known as neutralizing, makes it possible to provide an initial compensation for the spurious capacitance of a microsensor. It consists, when a carrier is injected into a microsensor, in adding to the signal delivered by the microsensor a so-called “neutralizing” signal to form an aggregate signal on which the effects of the spurious capacitance are cancelled or very much reduced. The neutralizing signal has a frequency that is identical to that of the carrier. It is also in phase opposition with the carrier. The amplitude of the neutralizing signal is fixed, determined from an estimation of the spurious capacitance that can result from computations, measurements or charts. The neutralizing signal can be generated, for example, by injection of a carrier in phase opposition with that injected into the microsensor, in a capacitor called a compensation capacitor, denoted CCompensation, the value of which is fixed and based on an estimation of the value of the spurious capacitance of the microsensor.
The value of the spurious capacitance of a microsensor can differ from one specimen of a microsensor to another, so that the estimation of the spurious capacitance requires prior measurements which threaten the truly industrial nature of the neutralizing function. Moreover, the value of the spurious capacitance can also vary according to the age of the microsensor, which makes it even more difficult to correct its effects.
FIG. 1 represents a signal acquisition device of a capacitive microsensor according to the state of the art and implementing a neutralizing method.
A microsensor 1 comprises a useful capacitance CUseful, the value of which varies in time, and a spurious capacitance CSpurious of fixed value. The variation in useful capacitance reaches, for example, an amplitude of 0.01 picoFarad (pF) at a frequency, for example, of 10 kHz. The spurious capacitance has a value, for example, equal to 10 pF.
A constant-amplitude carrier HF1 is sent to the input of the microsensor, a carrier HF2 is collected at the output of the microsensor and we try to assess the effects of the time variations of the useful capacitance Cuseful on the amplitude of the carrier HF2 by negating the effects of the spurious capacitance Cspurious on the carrier HF2. For this, a neutralizing signal SNeutralizing is used, as described above.
The neutralizing signal SNeutralizing is constructed from the carrier HF1, an inverter device 2, and a compensation capacitance CCompensation. The signal at the output of the inverter has an amplitude identical to that of the signal placed at the input of the inverter, it is in phase opposition with the signal with signal placed at the input of the inverter and is denoted HF1.
The neutralizing signal SNeutralizing is sent to a first input of a summing device 5, the carrier HF2 is sent to a second input of the summing device which delivers to an output an aggregate signal SAggregate comprising a sum of the carrier HF2 and of the neutralizing signal SNeutralizing. The aggregate signal SAggregate supplies a first inverting input of an amplifier 11, a second non-inverting input of the amplifier being linked to electrical ground. One output of the amplifier is linked to the inverting output through a parallel bridge comprising a load resistor RLoad and a load capacitor CLoad. An amplified aggregate signal is delivered by the output of the amplifier which is low impedance.
Then, the amplified aggregate signal is demodulated, that is, it is multiplied by the carrier HF1 by means of a multiplier 20. The signal delivered by the multiplier 20 is filtered by means of a low-pass filter 30 to form a baseband signal. Finally, the baseband signal is sampled and digitized, for example at a rate of 100 KHz, in the case of a carrier HF1 at the frequency of 250 kHz. The sampling and digitizing device can be, for example, a delta-sigma-type converter which delivers a bitstream comprising, for example, signed bits, at a rate, for example, equal to 100 MHz, and forming a digital measurement of the required amplitude modulation.
This description reveals other drawbacks presented by the signal acquisition devices belonging to the family of “charge amplification” devices of the state of the art. The acquisition device of the state of the art described, acts very much in the majority on analog signals, which poses problems regarding the choice and procurement of critical analog components comprising the acquisition device. Such is the case, for example, for the amplifier 11 and the multiplier 20 which must both be extremely powerful components which are therefore relatively costly. Moreover, it is difficult to integrate these components in an ASIC.